An externally heated carbonization furnace (an externally heated pyrolysis gasification furnace) is intended to modify a low-calorie substance (a low-grade substance) having high moisture content. The externally heated carbonization furnace produces a carbide with an improved calorific power, by indirectly heating mainly sewage sludge, woody biomass, low-grade coal, or the like at high temperatures ranging from 300° C. to 700° C. under the condition in which oxygen is cut off.
Known examples of a method for producing carbide include high-temperature carbonization in which a treated object is indirectly heated at high temperatures ranging from 500° C. to 700° C., and semi-carbonization (torrefaction) in which the treated object is indirectly heated at temperatures around 300° C. With the high-temperature carbonization, securing a sufficient treatment time under a predetermined temperature makes it possible to achieve carbide production that suppresses a high gasification rate and self-heat generation. With the semi-carbonization, controlling the temperature within an extremely narrow range with respect, in particular, to woody biomass makes it possible to achieve carbide production that strikes a balance between pulverizability and the residual ratio of heat quantity.
Further, known examples of the externally heated carbonization furnace include an externally heated rotary kiln that includes a kiln inner cylinder that rotates about an axis thereof and an outer cylinder that circulates heating gas around the kiln inner cylinder. The externally heated rotary kiln carries out a heat treatment while transferring the treated object (low-calorie substance) in the axial direction inside the kiln inner cylinder. Another known example is an externally heated rotary kiln divided into a former stage and a latter stage, which a treated object is dried in the former stage and carbonized in the latter stage (see Patent Document 1).